The present invention relates to semiconductor processing and, more particularly, to thermal reactors for chemical vapor deposition, thermal annealing, and other procedures requiring high temperature processing. A major objective of the present invention is an improved thermal reactor for semiconductor processing which provides for high throughput, flexible thermal control, and process uniformity for large wafers at ambient and reduced pressures.
Recent technological progress is closely identified with the increasing miniaturization of electronic circuits made possible by advances in semiconductor processing. Certain advanced processing techniques require exposing a semiconductor structure to a reactant gas under carefully controlled conditions of elevated temperature, sub-ambient pressures, and uniform reactant gas flow. Examples of such processes include low-pressure chemical vapor deposition, reduced-pressure chemical vapor deposition, and selective epitaxial deposition.
The economics of integrated circuit manufacture have favored increasing wafer diameters. Early wafers had diameters of 2" and less, while the trend today is to 8" and above. The trend toward larger wafer sizes is moderated by the added difficulty of obtaining uniform gas flow and heating required for high yields. Uniform gas flow is addressed by careful selection of reaction chamber geometry and the manner in which gases are introduced into and exhausted from the chamber; temperature uniformity has been addressed by supporting wafers with large thermal susceptors which distribute heat evenly over time and space. A major problem with large mass susceptors is the time they require to be heated and cooled. This increased time adversely affects wafer throughput and, thus, manufacturing costs.
A reactor system for implementing ambient and sub-ambient pressure thermal reactions typically includes a reactor vessel, a gas source, an exhaust system, a heat source, and a coolant source. The reactor vessel, which typically has quartz walls, defines a reaction chamber which serves as a controlled environment for the desired reaction. The gas source provides purging and reactant gases, while the exhaust system removes spent gases and maintains the desired ambient and sub-ambient pressures. The heat source, which can be an array of infrared lamps or an inductive source, generally transmits energy through the chamber wall to heat the wafer. Generally, the wafer is mounted on a support structure, which can serve as a susceptor to absorb energy transmitted into the chamber and conduct the resulting heat to the wafer being processed. In some reactor systems, the support can rotate the wafer within the chamber to minimize the effect of spatial anomalies within the chamber. Coolant can be directed against the chamber wall to minimize its thermal expansion and distortion. Coolant is also used after processing to speed the return of the reactor to room temperature.
Reaction chamber walls are typically cylindrical over a substantial portion of the chamber length, e.g., as in a bell jar chamber. Cylindrical vessels are the vessels of choice for low pressure applications since they distribute stress due to pressure differential uniformly and do so in spite of variations in wall thickness. The even distribution of stress minimizes the probability of breakage.
Ambient pressure thermal reactors exist which use rectangular quartz chambers to provide uniform reactant gas flow across a wafer. However, these are not reduced pressure or low pressure capable. The pressure differential across a flat surface would cause localized stress and subject it to breakage.
Stressing of flat and other non-cylindrical walls due to pressure differential can be addressed by using greater wall thicknesses or other reinforcement means. However, thick walls provide too much thermal insulation. The effectiveness of the external coolant, typically air, in reducing chamber wall temperatures would be reduced, leading to increased chemical deposition on the inner surfaces of the wall. Additional design objectives result in conflicting preferences between thin chamber walls, to reduce wall deposits, versus thicker walls to reduce pressure-caused stresses.
Susceptors, which are typically of graphite, often contain silicon from previous depositions. This silicon can migrate to the backside of a wafer when the susceptor is hotter than the wafer. This backside migration is generally considered undesirable. For example, backside transport is that it can disturb a dielectric seal previously applied to a wafer. In some cases, backside migration can be used advantageously to seal dopant in a wafer.
In most thermal reactors, backside migration is limited by heating the wafer and susceptor slowly so that no large temperature differentials are formed. Thus, control of backside migration, in addition to the requirement of uniform temperatures, imposes limitations on the rate at which a thermal reactor can be heated to a desired processing temperature. Furthermore, conventional reactors do not permit the relative temperature of wafer and susceptor to be varied precisely. Therefore, it is difficult to implement backside migration selectively so that dopant sealing can be implemented when desired.
What is needed is a thermal reactor system for semiconductor processing which provides for high-throughput thermal processing of large wafers. The high throughput must be achieved in connection with gas flow uniformity and thermal uniformity for high yields. Adjustment of relative wafer and susceptor temperatures, and thus control of backside migration, should be provided. Both ambient and reduced pressure processing capability are desired.